1. Field of the Invention
The present invention relates to a power supply unit and an electronic timepiece.
2. Background Art
Many small electronic devices often use batteries as a power supply, and thus, power saving is important. In such a small electronic device, the voltage of a battery is divided or stepped down so that appropriate voltages are supplied to the respective circuits. FIG. 12 is a block diagram of a power supply unit 900 of an electronic device having a TN (Twisted Nematic) liquid crystal. In FIG. 12, a TN liquid crystal 921 is a load. A battery 901 is a button cell, for example, of which the initial voltage is 3 V.
The voltage step-down circuit 902 which is a halver circuit steps down the voltage of the battery 901 to generate 1.2 V and supplies the generated voltage to an oscillator power supply circuit 903 and a LCD driver power supply circuit 905. The oscillator power supply circuit 903 supplies 0.9 V to an oscillator circuit 904 using the voltage of 1.2 V supplied from the voltage step-down circuit 902. Thus, the oscillator power supply circuit 903 consumes power corresponding to a voltage difference ΔV=0.3 V (0.3=1.2−0.9) between the input and the output.
The oscillator circuit 904 generates a clock signal used in a small device.
The LCD driver power supply circuit 905 which is connected to capacitors 907 to 909 steps up the input voltage of 1.2 V using the capacitors 907 to 909 to generate a voltage VL1=1.2 V which is 1 times the voltage of 1.2 V, a voltage VL2=2.4 V which is 2 times the voltage VL1, and a voltage VL3=3.6 V which is 3 times the voltage VL1. The LCD driver power supply circuit 905 supplies the generated voltages VL1=1.2 V, VL2=2.4 V, and VL3=3.6 V to an LCD driver circuit 906.
The LCD driver circuit 906 drives the TN liquid crystal 921 using the supplied voltages VL1=1.2 V, VL2=2.4 V, and VL3=3.6 V.
That is, when the TN liquid crystal 921 is a load, a voltage stepped down by the voltage step-down circuit 902 is supplied to the oscillator power supply circuit 903 and the LCD driver power supply circuit 905. Moreover, since the voltage step-down circuit 902 steps down the battery voltage to 1.2 V, for example, the voltage of the battery 901 can be used in the range of from 3 V (initial state) to 2.4 V.
FIG. 13 is a block diagram of a power supply unit 910 of an electronic device having a PN (Polymer Network) liquid crystal. In FIG. 13, a PN liquid crystal 922 is a load.
A battery 901 is a button cell, for example, of which the initial voltage is 3 V.
An oscillator power supply circuit 903 supplies a voltage of 0.9 V to an oscillator circuit 904 using the voltage ranging from 3 V to 2 V supplied from the battery 901.
The oscillator circuit 904 generates a clock signal used in a small device. Thus, the oscillator power supply circuit 903 consumes power corresponding to a voltage difference ΔV=2.1 V (2.1=3−0.9) to 1.1 V (1.1=2−0.9) between the input and the output.
An LCD driver power supply circuit 915 generates a voltage VL1=1.5 V using the input voltage ranging from 3V to 2V. The LCD driver power supply circuit 915 which is connected to capacitors 917 to 919 steps up the voltage VL1=1.5 V using the capacitors 917 to 919 to generate a voltage VL2=3.0 V which is 2 times the voltage VL1, and a voltage VL3=4.5 V which is 3 times the voltage VL1. The LCD driver power supply circuit 915 supplies the generated voltages VL1=1.5 V, VL2=3.0 V, and VL3=4.5 V to an LCD driver circuit 916.
The LCD driver circuit 916 drives the PN liquid crystal 922 using the voltages VL1=1.5 V, VL2=3.0 V, and VL3=4.5 V supplied from the LCD driver power supply circuit 915.
That is, when the PN liquid crystal 922 is a load, the maximum required driving voltage (for example, 4.5 V) is higher than that of the TN liquid crystal 921. Thus, when the PN liquid crystal 922 is a load, the voltage of the battery 901 is directly supplied to the oscillator power supply circuit 903 and the LCD driver power supply circuit 915. Moreover, since the LCD driver power supply circuit 915 steps down the battery voltage to 1.5 V, for example, the voltage of the battery 901 can be used in the range of from 3 V (initial state) to 2 V.
In a power supply unit of such a small electronic device, according to the related art disclosed in JP-A-6-327236, stepping up of voltage is realized by charging and discharging capacitors at a predetermined timing.
Moreover, in such a power supply unit, according to the related art disclosed in JP-A-57-76615, depending on a load state of a liquid crystal display unit, voltage is supplied from a battery to a constant voltage circuit under a heavy load state, whereas voltage is supplied from a voltage step-down circuit to the constant voltage circuit under a load state other than the heavy load state. In this way, when the liquid crystal display unit is under the heavy load state so that the output voltage of the battery decreases, the output of the constant voltage circuit is supplied to a logic unit.
However, in the related art, when the voltage step-down circuit shown in FIG. 12 is used, if the battery voltage decreases to 2.4 V, the stepped-down voltage will become 1.2 V which is ½ of the battery voltage. In this case, since the LCD driver power supply circuit 905 steps up the voltage of 1.2 V by 3 times to generate the voltage of 3.6 V, it is possible to drive the TN liquid crystal. However, in this case, the LCD driver power supply circuit 905 cannot drive the PN liquid crystal of which the maximum driving voltage is 4.5 V. Thus, in order to drive the liquid crystal, it is necessary to select and use any one of the power supply units shown in FIGS. 12 and 13 depending on the maximum driving voltage. If the power supply unit of FIG. 13 is selected, there is a problem in that the power consumed by the oscillator power supply circuit 903 is greater than the configuration of FIG. 12.
Moreover, in the related art disclosed in JP-A-6-327236, there is a problem in that capacitors for decreasing the power consumed by a voltage step-up circuit and a circuit for controlling the charging/discharging timings are required.
Moreover, in the related art disclosed in JP-A-57-76615, the voltage supplied to the liquid crystal display unit is changed depending on the load state of the liquid crystal display unit. Thus, when the PN liquid crystal which requires a high driving voltage is a load, if the battery voltage decreases, it is not possible to generate the maximum driving voltage required for driving. As a result, there is a problem in that when the PN liquid crystal is the load, stepping-up of voltage based on the voltage stepped down by the voltage step-down circuit is not performed, so that power consumption is not decreased.